Structural and Conformational Considerations

Structural and Conformational Considerations.—The gas-phase Raman spectrum of cyclopentane has been investigated whence it was found that the equilibrium out-of-plane amplitude is 0.47 0.025 A. The barrier to planarity, claimed to be the first direct spectroscopic measurement, is 5.21 0.14 kcal mol in agreement with the earlier value of 5kcalmol due to Pitzer. 

It is concluded from a study of the microwave spectrum of methylenecyclo-pentane that the ring is permanently twisted a barrier to pseudorotation of 1.8 kcal mol is found. This value is similar to that obtained for cyclo-pentanone, hence the two five-membered rings are of comparable rigidity. 

The crystal and molecular structures of racemic and (-h)-l,2-traMS-cyclo-pentane carboxylic acids have been determined by A -ray analysis. In the racemic and optically active series the internal rotation angle / C-l—C-2 has values of 40° and 41°, respectively, compared with a maximum value of 42°. This serves to bring the COjH groups as nearly equatorial as possible. 

For a non-planar regular pentagon the out-of-plane deformations can be described in terms of only two parameters and are given by 

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The next three sections (Sections 7.7.1, 7.7.2, and 7.7.3) cover fluorescence spectroscopy, I15-18 infrared, and circular dichroism, three powerful approaches to characterize the structure and conformational considerations of synthetic peptides. Section 7.7.1 deals with the use of fluorophores and broad aspects of fluorescence spectroscopy to characterize conformational aspects of peptide structure. In a similar manner, Section 7.7.2 covers a broad aspect of the uses of infrared (IR) techniques to study peptide conformations 19-22 Many IR techniques are discussed, as are approaches for the study of specific peptidic structures including amyloid, p-turn, and membrane peptides. Finally, there is a section on circular dichroism (Section 7.7.3) that covers the major issues of concern for peptide synthetic chemists such as the assignments of a-helix, 310-helix, -sheets and P-turns, and polyproline helices 23-25 There is also a brief description of cyclic peptides. 

Tartar, A. Demarly, A. Sergheraert, C. Escher, E. Structural and Conformational Considerations Angiotensin. In Proc in of the 8th American Peptide Symposium Hruby, V. J., Rich, D. H., Eds. Pierce Chemical Rockford, 1983 pp 377-380. 

The structures of four-membered rings are of considerable interest, owing in part to the low-frequency ring puckering vibration203. The comparison of the structures and conformational preferences of thietane oxides and dioxides discussed above with those of dithietane oxides and dioxides is therefore appropriate and will follow. 

The parent fulvene, 5-methylene-1,3-cyclopentadiene, was the subject of numerous calculations and conformational considerations. Both structures derived from microwave... 

A considerable number of nickel(II) complexes do not behave consistently in accord with expectations for any discrete structural types, and they have in the past been termed anomalous. All the anomalies can be satisfactorily explained in terms of several types of conformational or other structural change and, ironically, so many examples are now known that the term anomalous is no longer appropriate. The three main structural and conformational changes that nickel(II) complexes undergo are described and illustrated next. 

While the structural and dynamic considerations described above indicate the basis for the mechanism of selectivity, a direct way to determine which DNA sequences are the least costly to bend is to calculate the free energy cost of the conformational transition. Unfortunately, this is not straightforward, due to the large change in conformation. Instead of calculating the cost of bending the entire TATA element, we have studied segments of the TATA box that are more amenable to calculations of this sort. 

The 9(10-> l9)-abeo analogue (6) of oestrone methyl ether has been shown by X-ray crystallography to have the normal 9a -configuration, but because of the chair conformation of the enlarged ring b the molecule differs considerably in shape from oestrone methyl ether. X-Ray, n.m.r., and o.r.d. data establish the structures and absolute configurations of a series of insect-repellent steroids isolated from Nicandra physaloides. The structure and conformation of the 6-sila-steroid (7) have been determined. ... 

The physicochemical properties of alkylated polysaccharides have received some attention, and details of the structures and conformations of 0-methylcelluloses, and their interaction in micelle junctions, have been included in a thorough discussion of polysaccharide gels and networks. Information on the polydispersity of samples of partially methylated cellulose may be obtained from column fractionation and by fractional precipitation from a solvent-nonsolvent system, but, for a more complete characterization of polydispersity, fractionation with a series of solvent-nonsolvent systems is necessary. The solubility, in water, of polysaccharides that are mainly methylated may be considerably improved by introduction of a few suitable ionizing groups, for example, by reaction with monochloroacetic acid to introduce carboxymethyl ether groups. The general sorption and diflFusion features of hydrocarbons and other... 

Consideration of the structural and conformational features which conspire to produce very large differences in the relative energies of diastereomeric transition states will be crucial to future studies. Asymmetric reduction may prove to be a sensitive probe for detecting subtle steric and electronic disparities in such transition states. More quantitative investigations are needed to test the potential of the method. 

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